What Are Astrocytes? Definition, Types, and Their Vital Functions
An astrocyte is a star-shaped cell. It is a type of neuroglia found in the nervous system in both vertebrates and invertebrates. Protoplasmic and fibrous astrocytes are two types of astrocytes. Fibrous astrocytes are more prevalent among the myelinated nerve fibres in the central nervous system's white matter. Also, the organelles in the somata of neurons are seen in the astrocytes, but they appear to be sparser. These cells are characterized in their cytoplasm by the presence of numerous fibrils. The primary processes exit the cell in a radial direction (thus, the name astrocyte means "star-shaped cell"), forming the expansions and endfeet at the vascular capillaries' surfaces.
Astrocytes Structure
Let us discuss the astrocytes structure. Astrocytes are the sub-type of glial cells of the central nervous system. They are also called astrocytic glial cells. Star-shaped, their several processes envelop synapses made by neurons. In humans, a single astrocyte cell may interact with up to 2 million synapses at one time. Histological research has traditionally been used to identify astrocytes; many of these cells express the intermediate filament Glial Fibrillary Acidic Protein (GFAP).
Many astrocyte forms exist in the central nervous system, including protoplasmic (in grey matter), fibrous (in white matter), and radial. Usually, the fibrous glia is located within the white matter, contains relatively few organelles, and exhibits long unbranched cellular processes. Often, this type has astrocytic end-foot processes, which physically connect the cells to the outside of capillary walls when they are in proximity to them. The protoplasmic glia is the most prevalent one and is found in grey matter tissue. They possess a larger quantity of organelles and exhibit short and highly branched tertiary processes.
The radial glial cells are disposed of in the planes that are perpendicular to the ventricle axes. One of their processes will abut the pia mater, where the other is deeply buried in the grey matter. Mostly, the radial glia is present during development, playing the main role in neuron migration. Müller cells of the Bergmann glia and retina cells of the cerebellar cortex are the only cells that survive into adulthood. Both three types of astrocytes send out processes to form the pial-glial membrane when they are near the pia mater.
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Astrocytes Function
Let us know about the astrocytes function in detail.
Astrocytes help form the physical structure of the brain and are thought to play numerous acting roles, including the absorption or secretion of the neural transmitters and the blood-brain barrier maintenance. The tripartite synapse concept has been proposed, referring to the tight relationship taking place at synapses among the presynaptic element, a glial element, and a postsynaptic element.
Structural: They are involved in the brain's physical structuring. Astrocytes get their name from the fact that they are shaped like stars. They are the most common glial cells in the brain, and they are closely linked to neuronal synapses. They control how electrical signals are transmitted in the brain.
Glycogen Fuel Reserve Buffer: Astrocytes have glycogen, and they are capable of gluconeogenesis. The astrocytes, which are next to neurons in the hippocampus, store and frontal cortex and release glucose. Therefore, astrocytes may fuel neurons with glucose during periods of high glucose consumption rate and glucose shortage. Recent research on rats suggests there can be a connection between physical exercise and this activity.
Protoplasmic Astrocytes
Protoplasmic astrocytes, unlike fibrous astrocytes, are found in the grey matter of the central nervous system. They have fewer fibrils in their cytoplasm and fewer cytoplasmic organelles, allowing their somata to be formed by the surrounding fibres and neurons. Also, the processes of protoplasmic astrocytes make contact with capillaries.
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Astrocytes divide after the nervous system injury and occupy spaces left by the injured neurons. Also, astrocytes are thought to contain high-affinity uptake systems for the neurotransmitters such as gamma-aminobutyric acid (GABA) and glutamate. This function is essential in the modulation of synaptic transmission since uptake systems tend to terminate the neurotransmitter action at the synapses and also can act as storage systems for neurotransmitters when they are required.
Clinical Significance
Astrocytomas
Astrocytomas are the primary intracranial tumors, which develop from astrocytes. Also, it is possible that neural stem cells or glial progenitors may give rise to astrocytomas. These tumors may take place in several parts of the brain and/or spinal cord. Astrocytomas are classified into two categories: low grade (I and II) and high grade (III and IV). Low-grade tumors are very common in children, whereas high-grade tumors are very common in adults. Malignant astrocytomas are prevalent among men, contributing to the worst survival.
Pilocytic astrocytomas are grade I tumors. They are considered slow-growing and benign tumors. The solid portion of pilocytic astrocytomas often contains cystic sections filled with a nodule and fluid. Many are located in the cerebellum. Thus, most symptoms are related to coordination or balance difficulties. They also take place more frequently in teens and children.
FAQs on Astrocyte: Key Roles, Structure, and Importance in Biology
1. What is an astrocyte?
An astrocyte is a star-shaped glial cell found in the central nervous system (CNS), which includes the brain and spinal cord. They are the most abundant type of glial cell and are also known as astroglia. They do not conduct nerve impulses but play a crucial role in supporting and protecting neurons.
2. What are the primary functions of astrocytes in the brain?
Astrocytes perform several vital functions essential for maintaining a healthy nervous system. Key functions include:
- Structural Support: They provide a physical framework for neurons within the CNS.
- Nutrient Supply: They connect neurons to blood vessels, helping to supply nutrients like glucose and oxygen.
- Neurotransmitter Regulation: They help clear excess neurotransmitters, such as glutamate, from the synaptic cleft to ensure precise signalling.
- Blood-Brain Barrier (BBB) Maintenance: They help induce and maintain the tight junctions of the BBB, which protects the brain from harmful substances.
- Injury Response: They respond to injury by forming a glial scar, a process known as astrogliosis, to contain damage.
3. What are the two main anatomical types of astrocytes?
Based on their structure and location, astrocytes are classified into two main types:
- Protoplasmic Astrocytes: These are found primarily in the grey matter. They have many short, thick, and highly branched processes that envelop synapses and neuronal cell bodies.
- Fibrous Astrocytes: These are typically located in the white matter. They have long, thin, and less branched processes that are often aligned with myelinated nerve fibres.
4. How do astrocytes differ from other glial cells like oligodendrocytes and microglia?
While all are glial cells in the CNS, their primary roles are distinct. Astrocytes focus on providing structural support, regulating the chemical environment, and maintaining the blood-brain barrier. In contrast, oligodendrocytes are responsible for producing the myelin sheath that insulates nerve axons, enabling faster signal transmission. Microglia function as the resident immune cells of the CNS, responsible for removing cellular debris and pathogens through phagocytosis.
5. How do astrocytes actively support synaptic function?
Astrocytes are not just passive support cells; they actively participate in synaptic activity through a concept known as the 'tripartite synapse' (pre-synaptic neuron, post-synaptic neuron, and astrocyte). They support synapses by:
- Uptaking excess neurotransmitters like glutamate and GABA from the synapse, which prevents neurotoxicity and ensures signal clarity.
- Releasing their own signalling molecules, called gliotransmitters (like ATP and D-serine), which can modulate neuronal excitability and synaptic transmission.
- Providing neurons with energy substrates, such as lactate, to meet the high metabolic demands of synaptic activity.
6. Why is the role of astrocytes in the blood-brain barrier (BBB) considered so critical?
The role of astrocytes is critical because they are essential for both the formation and maintenance of the BBB. While the barrier itself is formed by the tight junctions between endothelial cells lining the brain's capillaries, astrocytes release factors that signal these cells to form and maintain these strong connections. Their 'end-feet' wrap around the capillaries, allowing them to regulate blood flow and the transport of substances like water and glucose into the brain, thus acting as gatekeepers for the neuronal environment.
7. What is the connection between astrocyte dysfunction and neurological disorders?
Dysfunctional astrocytes, a condition known as astrocytopathy, are implicated in many neurological and psychiatric disorders. For example, in conditions like Alzheimer's disease or after a stroke, astrocytes can become reactive (astrogliosis), releasing inflammatory molecules that can harm neurons. In disorders like epilepsy, their failure to properly buffer potassium ions and glutamate can lead to neuronal hyperexcitability and seizures. Their malfunction during development is also linked to neurodevelopmental disorders like autism spectrum disorders.
8. What is an astrocytoma?
An astrocytoma is a type of brain tumour that originates from astrocytes. These tumours can vary widely in their aggressiveness. Low-grade astrocytomas are slow-growing and relatively contained, while high-grade astrocytomas, such as anaplastic astrocytoma and glioblastoma, are malignant, grow rapidly, and are difficult to treat because they invade surrounding brain tissue.
